題意： 字母變成大小寫，數字不變
Examples:
Input: S = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]

Input: S = "3z4"
Output: ["3z4", "3Z4"]

Input: S = "12345"
Output: ["12345"]


S = a1bc, 畫出遞迴數如下：如果是字母 就是一個二叉樹， 如果是數字就一個節點。所以本質上是一個二叉樹的back tracking.

因為不熟悉 Java 字串處理的一些function, 一開始寫了一個特別醜陋的code:

class Solution {
    
 
public List<String> letterCasePermutation(String S) {
        
        List<String> result = new ArrayList<>();
        StringBuilder sb = new StringBuilder(S);
        dfs(new StringBuilder(), sb, result,0);
        return result;
    
    }
    
    private void dfs(StringBuilder curResult, StringBuilder S, List<String> result, int
 
 index){
        
          if(curResult.length() == S.length()){
              result.add(curResult.toString());
              return;
          }
           
          char c = S.charAt(index);  
          curResult.append(c);
          dfs(curResult,S,result,index+1);
          curResult.setLength(curResult.length()
 
-1);   
        
          if(c>='a' && c<='z'){
              
             char ch = change_char(c, true);  
             S.setCharAt(index,ch);  
             curResult.append(ch);  
             dfs(curResult,S,result,index+1);
             curResult.setLength(curResult.length()-1); 
             S.setCharAt(index,c);  
           }  
        
           else if(c>='A' && c<='Z'){    
              char ch = change_char(c,false);
              S.setCharAt(index,ch); 
              curResult.append(ch);   
              dfs(curResult,S,result,index+1);
              curResult.setLength(curResult.length()-1);
              S.setCharAt(index,c);   
          }      
    }
    
    private char change_char(char c, boolean small){
        if(small){
            return (char)(c + 'A'-'a');
        }
        else return  (char)(c + 'a'-'A');
    }
}

用了Character 類裡的function 後的code:

優化到了95%

class Solution {
    public List<String> letterCasePermutation(String S) {
        
        List<String> result = new ArrayList<>();
        dfs(new StringBuilder(), S, result,0);
        return result;
    
    }
    
    private void dfs(StringBuilder curResult, String S, List<String> result, int index){
        
          if(curResult.length() == S.length()){
              result.add(curResult.toString());
              return;
          }
           
          Character c = S.charAt(index); 
        
          if(Character.isLetter(c)){
              // left sub tree
              curResult.append(Character.toLowerCase(c));
              dfs(curResult,S,result,index+1);
              curResult.deleteCharAt(curResult.length()-1);
              
              //right sub tree
              curResult.append(Character.toUpperCase(c));
              dfs(curResult,S,result,index+1);
              curResult.deleteCharAt(curResult.length()-1);  
          }
        
          else{ // is number
              curResult.append(c);
              dfs(curResult,S,result,index+1);
              curResult.deleteCharAt(curResult.length()-1);
          }    
    }
}